A subframe is a structural component of a vehicle, such as an automobile or an aircraft, that uses a discrete, separate structure within a larger body-on-frame or unit body to carry certain components, such as the engine, drivetrain, or suspension. An “isolated” subframe refers to a subframe that is a separate structure that attaches to the vehicle body. The subframe can be bolted and/or welded to the vehicle. When bolted, it is sometimes equipped with rubber bushings or springs to dampen vibration.
Body-on-frame is an automobile construction technology. Mounting a separate body to a rigid frame that supports the drivetrain was the original method of building automobiles, and its use continues to this day. Unit body is a construction technique that uses the external skin of an object to support some or most of the load on the structure.
The principal purpose of using a subframe is to isolate vibration and harshness from the rest of the body. For example, in an automobile with its powertrain contained in a subframe, forces generated by the engine and transmission can be damped enough that they will not disturb passengers. Using rubber bushings helps to dampen vibration.
Subframes are typically mounted at the bottom of a vehicle body and can be located at the front and/or the rear of a vehicle body. In a front-engine, rear-drive type of vehicle, front suspensions and an engine are mounted on a front subframe at the front side of the vehicle body, and rear suspensions and a differential gear unit are mounted on a rear subframe at the rear side of the vehicle body.
Precision in locating subframes to the vehicle body is required for maintaining critical driveline angles. Such precision may not be achievable with commonly-used mounting techniques and hardware, particularly when rubber bushings are employed. A vehicle's driveline consists of the parts of the drivetrain excluding the engine and transmission. It is the portion of a vehicle, after the transmission, that changes depending on whether a vehicle is front wheel drive, four wheel drive, all wheel drive, or rear wheel drive.
The driveline angle is the angle between the driveshaft centerline and the rear differential input shaft centerline, and is often referred to as “critical” because it affects driveline vibration. Deviant driveline angles can cause driveline vibrations, as well as driveline component durability concerns. Driveline angle is also used to offset a vehicle's transmission-to-driveshaft angle to phase the U-joint of the CV-joint appropriately to reduce velocity variation.
Known methods for precisely locating isolated subframes to the body include fixed pins located on an assembly plant decking fixture. Typically, the decking fixture holds and transports the suspension module (including a subframe) to the vehicle body, and then lifts the suspension module to the vehicle body for the decking process. The fixed pins are generally only acceptable when the assembly plant is dedicated to building a single vehicle platform, because they would likely be incompatible with any other platform's body structure and suspension/driveline system. This would be particularly true if an incremental, low-volume vehicle platform required fixed pins that would interfere with the resident vehicle platform's body structure. Compatibility would require a change in the body design or tooling on the resident vehicle platform, which would be undesirable. While the fixed pins could be tooled to fold down or collapse to accommodate different vehicle platforms, modifying existing decking fixtures with collapsible tooling for incremental, low-volume vehicles would be cost prohibitive.
FIG. 1 illustrates a prior art decking and subframe assembly used in vehicle manufacturing. As can be seen, the vehicle platform or assembly plant decking fixture includes a decking pallet. A vehicle subframe is located on the decking pallet via fixed decking pallet-to-subframe alignment pins. In this prior art embodiment, four fixed decking pallet-to-subframe pins are utilized. The decking pallet includes additional fixed pins, such as decking pallet-to-vehicle alignment pins. Using this prior art embodiment, both the subframe and the vehicle are aligned with respect to the decking pallet, and thus are aligned with respect to each other.
For vehicles utilizing isolated subframes, such as rear wheel drive and all wheel drive vehicles, locating the rear subframe to the body with precision is required for maintaining critical driveline angles. Subframes typically include four mounting fasteners and bushings, which are used to locate the subframe on the vehicle body when the subframe is attached to the vehicle body. The mounting fasteners and bushings, however, typically do not have a tight enough tolerance to properly control the critical driveline angles. Such tolerances are preferably within a degree, but may vary, for example, by vehicle.
Many vehicle platforms increase tolerance with pins (for accurate plan-view location/orientation) that are attached to the assembly plant decking fixture, where the pins route through more precise holes in the isolated subframe and body rail. In the vehicle plan view, the rear subframe/rear suspension needs to be located and decked “square” to the vehicle as closely as possible. If it is slightly rotated when decked to the vehicle, the driveshaft-to-axle angle will be affected and the rear suspension will not track straight ahead. With tight tolerance, the alignment of these holes will locate the isolated subframe accurately enough to support the driveline angle specifications for a given vehicle. The subframe fasteners are then torqued, and the decking fixture is lowered, withdrawing the pins from the body and the subframe.
Another strategy is to precisely locate the subframe on a “moon-buggy” decking fixture such as the that illustrated in FIG. 1. This fixture is then accurately located to the vehicle body via additional pins. Both of these methods use multiple fixed pins on the decking fixture to align the subframe and the vehicle body. A moon buggy decking fixture, as used herein, refers generally to a decking fixture that moves within an assembly plant, such as a decking fixture that transports a suspension module or powertrain module to the vehicle body and then lifts the module to the vehicle body for the decking process. Moon buggy decking fixtures are also referred to as pallet decking fixtures. These decking fixtures can vary greatly in design.
As can be seen from FIG. 1, the fixed pins on the decking pallet would most likely be incompatible with another platform's body structure and suspension/driveline system, and would not allow much platform flexibility for the assembly plant.
Further features of the present invention, as well as the structure of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.